The present invention is directed to methods for treating a conductive aluminum substrate and to aluminum substrates treated according to the present methods. The present invention is also directed to methods of forming photoconductors wherein the photoconductor aluminum substrate is treated in accordance with the aforementioned methods, and to photoconductors formed by such methods.
Typically, photoconductors comprise a conductive substrate which is conventionally formed of aluminum. It is advantageous to anodize the surface of the conductive aluminum substrate in order to improve the toughness and handling ability of the substrate and to suppress specular reflections such as Moire patterns. In a typical anodization process, the raw aluminum substrate or core is first cleaned and deoxidized, and then a porous alumina/hydrated aluminum oxide layer is formed by electrolytic oxidation of the aluminum in an electrochemical cell. The alumina layer is highly porous as it generally comprises hexagonal columns separated by deep pores.
The porous alumina layer is usually sealed, i.e., the pores of the alumina are filled or closed, in order to improve the electrophotographic properties of the anodized substrate. Several different sealing processes are commonly used. One sealing process, referred to as the water sealing process, involves immersion of the porous alumina layer in boiling water to convert the alumina to a hydrated alumina phase. This conversion is accompanied by a volume increase which seals or plugs the pores. In another sealing process, a metallic salt, for example a salt of a heavy metal such as cobalt or nickel, is contacted with the alumina layer. The metal deposits, typically as a hydroxide, within the pores of the alumina layer to provide a sealing effect. However, the use of heavy metals such as cobalt or nickel is disadvantageous in that high waste disposal costs are incurred. Additionally, alumina surface layers sealed by either the water sealing or metal salt sealing processes exhibit less than optimal adhesion to overlying layers when the aluminum substrates are employed in photoconductors. Poor adhesion of photoconductive layers to an aluminum substrate can result in catastrophic delamination during printer operation and therefore an undesirably shortened useful life for the photoconductor.
In the past, barrier layers and/or sublayers have been employed between a photoconductor substrate and an adjacent charge generation layer or charge transport layer. However, the use of such layers is disadvantageous in that the barrier or sublayer must be applied by dip coating or another controlled process and therefore significantly increases both the production time and costs for the photoconductor.
Accordingly, a need exists for methods for more easily preparing aluminum substrates, for example for use in photoconductors and for photoconductor substrates, which exhibit improved adhesion to overlying photoconductive layers.
Accordingly, it is an object of the present invention to provide methods for treating conductive aluminum substrates. It is a more specific object of the present invention to provide methods for treating conductive aluminum substrates which are suitable for use in photoconductors. It is a further object of the invention to provide methods for treating aluminum photoconductor substrates to provide the substrates with good toughness and handling ability while overcoming disadvantages of prior art methods. It is a related object to provide conductive aluminum substrates which exhibit good adhesion to overlying layers when the substrates are employed as photoconductor substrates, while maintaining good performance of the photoconductors.
These and additional objects and advantages are provided by the methods, substrates and photoconductors of the present invention. According to the present invention, the methods for treating a conductive aluminum substrate comprise anodizing a surface of an aluminum substrate to form a porous alumina surface layer, contacting the alumina surface layer with a liquid dispersion or solution of a polymer or at least one polymer-forming component, under conditions sufficient for the polymer to seal pores of the alumina surface layer, and removing excess polymer from the alumina surface layer. Preferably, the excess polymer is removed from the alumina surface layer by rinsing. Optionally, the surface of the treated alumina layer may be dried. The polymer seals the pores of the alumina surface layer to allow good electrophotographic properties when the layer is employed as a photoconductor substrate and allows good adhesion of the substrate to overlying photoconductive layers, for example overlying charge generation layers and/or charge transport layers used to form photoconductors. Additionally, photoconductors which include conductive aluminum substrates treated according to the present methods exhibit good electrical characteristics and print quality and improved durability owing to the good adhesion of the photoconductive layers to the aluminum substrate.
These and additional objects and advantages provided by the methods, substrates and photoconductors of the present invention will be more fully understood in view of the following detailed description.
According to the present methods, the surface of a conductive aluminum substrate is anodized to form a porous alumina surface layer. Preferably, the surface is cleaned and deoxidized prior to the anodization. The alumina surface layer is then contacted with a liquid dispersion or solution of a polymer or at least one polymer-forming component under conditions sufficient for polymer to seal pores of the alumina surface layer. Excess polymer is then removed from the alumina surface layer, for example by rinsing the layer before the polymer dries thereon.
The conductive aluminum substrate is described herein as suitable for use as a substrate of a photoconductor. However, it will be apparent to those skilled in the art that the conductive aluminum substrate as disclosed herein may be employed in various other devices and embodiments. Typically, a photoconductor substrate is in the form of a drum, and comprises a thin surface layer of aluminum which functions as an electrical ground plane. The aluminum may be deposited on the drum by any suitable method, including, for example, by vacuum evaporation. Both aluminum and aluminum alloys may be employed. Various aluminum alloys are suitable for preparing photoconductive drum substrates, one example of which comprises the alloy 3003. The aluminum substrate will have a thickness adequate to provide the required mechanical stability for the photoconductor. Typically, drum substrates have a thickness of from about 0.75 mm to about 1 mm, although greater or smaller thicknesses are equally within the scope of this invention.
Processes for anodization of a surface of an aluminum substrate to form a porous alumina surface layer are known in the art and may be employed in the present methods. Preferably, the aluminum is first cleaned and deoxidized before anodization. To anodize the surface of the aluminum layer, the aluminum is subjected to electrolytic oxidation in an electrochemical cell. The electrolyte typically comprises an acidic component, for example an inorganic acid such as sulfuric acid, although many other acids may be employed in place of sulfuric acid in accordance with techniques known in the art. The alumina grows as hexagonal columns separated by deep pores resulting in an alumina layer which is highly porous. The anodization may be conducted to form an alumina layer of any desired thickness. When the aluminum substrate is to be employed as a photoconductor substrate, an alumina surface layer of up to about 10 microns, preferably up to about 5 microns, is preferred. Suitable cell operating conditions, including specific electrolyte composition and concentration, bath temperature, current density and duration, will be apparent to one of ordinary skill in the art and selected depending on the desired thickness of the alumina surface layer. The resulting alumina layer is advantageous in that it improves the toughness and handling ability of the aluminum substrate and substantially suppresses specular reflections such as Moire patterns.
However, because the alumina layer is porous, the pores must be filled or sealed to allow good electrophotographic properties when the alumina layer is employed as a photoconductor substrate. Conventional methods for sealing the alumina layer generally cause the substrate to exhibit relatively poor adhesion to overlying layers. Previous steps to improve adhesion of the substrate to overlying photoconductive layers have encountered various disadvantages, for example in increasing the processing time and costs and/or in providing substrates which disadvantageously effect photoconductor properties such as electrical performance and/or print quality. However, such disadvantages are overcome and good substrate adhesion is provided by the methods of the present invention wherein the anodized aluminum substrate containing an alumina surface layer is contacted with a liquid dispersion or solution of a polymer or at least one polymer-forming component under conditions sufficient for polymer to seal pores of the alumina surface layer. Although the present invention is not intended to be limited by theory, it is believed that the polymer is adsorbed into the alumina pores and thereby seals the pores of the alumina surface layer. Excess polymer is removed from the polymer surface layer, for example by rinsing, before the polymer dries thereon. It is believed that the adsorption of the polymer into the alumina pores is substantially irreversible in the absence of additional removal techniques, whereby the rinsing removes all polymer except that adsorbed and held in the pores by surface attraction.
Various polymers are suitable for use in the methods of the present invention. Additionally, polymer-forming components which form polymers in situ in the liquid dispersion may also be employed. As will be discussed in further detail below, phenoxy resins, epoxy resins and polyalkylene oxide polymers, for example polyethylene oxide, polypropylene oxide and the like, are particularly suitable for use in the present methods. Generally, epoxy resins are formed from an epoxy compound such as epichlorohydrin and contain epoxide groups before crosslinking. Commonly, epoxy resins are formed by condensing epichlorohydrin with bisphenol A, preferably using an excess of epichlorohydrin in order to ensure epoxide groups are contained at each end of the resulting polymer. Such epoxy resins are generally of the following formula: 
Epoxy resins suitable for use herein include, but are not limited to butadiene-acrylonitrile modified epoxy resins, urethane modified epoxy resins, epoxy modified novolac resins, and the like. Epoxy resins are particularly suitable for use in the treatment methods of the invention as they dry to a clear film on the alumina surface layer.
Similarly, phenoxy resins are well known in the art and are commercially available from various sources. Phenoxy resins are commonly formed from epichlorohydrin and bisphenol A, but do not contain epoxy groups. The phenoxy resins generally have a repeating unit of the following formula: 
Naturally occurring polymers, for example pectin, are also suitable for use in the methods of the present invention. Pectin may be employed in its naturally occurring form or the pectin may be functionalized, for example with a silicon-containing organic compound, such as N-[3-triethoxysilylpropyl]-4,5-dihydroimidazole (TSPI).
Various polymer-forming components may also be employed in the liquid dispersion or solution to form in situ a polymer for sealing pores of the alumina. An example of such a polymer-forming component comprises TSPI, although other suitable components will be apparent to those skilled in the art. Within the scope of the present specification, reference to a liquid dispersion or solution of a polymer-forming component refers to a liquid dispersion or solution to which a polymer-forming component has been added, as those skilled in the art will recognize that upon addition of the polymer-forming component, in situ polymerization of the polymer-forming component or derivative thereof may proceed to such an extent that the added component may not exist in the solution in its original form.
In accordance with an important feature, the polymer or polymer-forming component or components are dispersed or dissolved in a liquid for contact with the alumina surface layer. The liquid may comprise water or a mixture of water and an organic liquid, or alternatively, may consist of one or more organic liquids. In a preferred embodiment, the alumina surface layer is contacted with an aqueous dispersion or solution of a polymer or at least one polymer-forming component. Surfactants or other additives may be added to the dispersion or solution as desired.
Suitably, the alumina surface layer may be contacted with the liquid dispersion or solution of polymer or polymer-forming component by immersing at least the alumina surface layer, and preferably the entire substrate, in the liquid dispersion or solution of the polymer or polymer-forming component. Immersion of the substrate facilitates this process step although other techniques, including spraying and the like may be employed as long as the application conditions are sufficient for the polymer to seal pores of the alumina surface layer. The present method is distinguishable from dip coating which has conventionally been employed to apply a barrier or sublayer to an aluminum substrate in that such conventional dip coating applications require that the substrate is removed from the coating bath at a controlled rate in order to form a uniform surface layer. To the contrary, in the present methods, the alumina surface layer does not require removal from the liquid dispersion or solution at a controlled rate. In fact, according to the present methods, excess polymer is removed from the alumina surface layer before the polymer dries on the surface layer.
The liquid dispersion or solution which is contacted with the alumina surface layer may be at any suitable temperature to facilitate sealing of the pores of the alumina by the polymer. Depending on the specific composition of the polymer, the liquid dispersion or solution may be at or below room temperature or alternatively may be at a temperature greater than room temperature, for example, greater than about 40xc2x0 C. or even greater than about 50xc2x0 C. Additionally, the concentration of the polymer or polymer-forming components in the liquid dispersion will be sufficient to allow the polymer to seal pores of the alumina surface layer. In a preferred embodiment, the liquid dispersion or solution contains greater than about 0.5 volume percent of the polymer of polymer-forming components and more preferably the liquid dispersion or solution contains from about 1 to about 10 volume percent of the polymer or polymer-forming components.
Typically, the alumina surface layer may be immersed in the liquid dispersion or solution for a period of time (the seal time) of at least about 30 seconds, and preferably greater than about one minute and less than about 60 minutes. More preferably, the seal time in which the alumina surface layer is contacted with the liquid dispersion or solution is in the range of from about three to about 10 minutes in order to obtain sufficient sealing of the porous alumina by the polymer.
Preferably, the excess polymer is removed from the alumina surface layer by rinsing, and more preferably by rinsing with an aqueous rinse liquid. The aqueous rinse liquid may be water only, or may be a combination of water and one or more organic rinse liquids. Alternatively, the excess polymer may be removed by rinsing with one or more organic rinse liquids, in the absence of water. Water is the preferred rinsing liquid. Other techniques for removing excess polymer from the alumina surface layer while maintaining the polymer which seals the pores of the alumina surface layer thereon, will be apparent to those skilled in the art.
The rinse liquid which is employed to remove excess polymer may be at any suitable temperature. In a preferred embodiment, the rinse liquid is at room temperature or a temperature greater than room temperature. As will be discussed in further detail in the examples set forth below, rinse temperatures greater than room temperature are particularly advantageous for use with certain polymers. Rinse liquid temperatures greater than about 50xc2x0 C., and even up to boiling temperatures, may be particularly preferred. If desirable, both a cold rinse (for example at room temperature) and a hot rinse at an elevated temperature may be employed to remove excess polymer. After rinsing, the aluminum substrate may be dried in order to expedite processing.
The aluminum substrates which are produced according to the present methods comprise an anodized surface layer of porous alumina and a polymer which seals the pores of the alumina surface layer. Since excess polymer is removed from the alumina surface layer, a polymer coating typically does not extend above the anodized alumina layer of the substrate. Accordingly, the thickness of any coating of the polymer which extends above the anodized alumina layer is not substantial and typically less than about 1 micron.
The substrates are particularly suitable for use as photoconductor substrates in accordance with well known photoconductor technology. In a preferred embodiment, a substrate treated according to the methods of the present invention serves as a substrate for a dual layer photoconductor which comprises a substrate, a charge transport layer and a charge generation layer.
In a preferred embodiment, the charge generation layer may be formed on the photoconductor substrate formed according to the present methods, followed by formation of the charge transport layer containing a hole transport compound, whereby a negative charge may be placed on the photoconductor surface. Conversely, the charge transport layer containing a hole transport compound may be formed on the photoconductor substrate and the charge generation layer is in turn formed on the charge transport layer, whereby a positive charge may be placed on the photoconductor surface. As one skilled in the art will appreciate, if the charge transport layer contains an electron transport material, the charges which may be placed on the photoconductor surface as a result of the arrangement of the charge transport and charge generation layers will be reversed.
The charge transport layer included in the dual layer photoconductors typically comprises binder and a charge transport compound. The charge transport layer is in accordance with conventional practices in the art and therefore may include any binder and any charge transport compound generally known in the art for use in charge transport layers. Typically, the binder is polymeric and may comprise, but is not limited to, vinyl polymers such as polyvinyl chloride, polyvinyl butyral, polyvinyl acetate, styrene polymers, and copolymers of these vinyl polymers, acrylic acid and acrylate polymers and copolymers, polycarbonate polymers and copolymers, including polyestercarbonates, polyesters, alkyd resins, polyamides, polyurethanes, epoxy resins and the like.
Conventional charge transport compounds suitable for use in the charge transport layer of the photoconductors of the present invention should be capable of supporting the injection of photo-generated holes or electrons from the charge generation layer and allowing the transport of these holes or electrons through the charge transport layer to selectively discharge the surface charge. Suitable charge transport compounds for use in the charge transport layer include, but are not limited to, the following:
1. Diamine and triarylamine transport molecules of the types described in U.S. Pat. Nos. 4,306,008, 4,304,829, 4,233,384, 4,115,116, 4,299,897, 4,265,990 and/or 4,081,274. Typical diamine transport molecules include N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(alkylphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamines wherein the alkyl is, for example, methyl, ethyl, propyl, n-butyl, or the like, or halogen substituted derivatives thereof, commonly referred to as benzidine and substituted benzidine compounds, and the like. Typical triarylamines include, for example, tritolylamine, and the like.
2. Pyrazoline transport molecules as disclosed in U.S. Pat. Nos. 4,315,982, 4,278,746 and 3,837,851. Typical pyrazoline transport molecules include 1-[lepidyl-(2)]-3-(p-diethylaminophenyl)-5-(p-diethylaminophenyl)pyrazoline, 1-[quinolyl-(2)]-3-(p-diethylaminophenyl)-5-(p-diethylaminophenyl)pyrazoline, 1-[pyridyl-(2)]-3-(1-diethylaminostyryl)-5-(p-diethylaminophenyl)pyrazoline, 1-[6-methoxypyridyl-(2)]-3-(-diethylaminostyryl)-5-(p-diethylaminophenyl) pyrazoline, 1-phenyl-3-[-p-diethylaminostyryl]-5-(p-dimethylaminostyryl)pyrazoline, 1-phenyl-3-[p-diethylaminostyryl]-5-(p-diethylaminostyryl)pyrazoline, and the like.
3. Substituted fluorene charge transport molecules as described in U.S. Pat. No. 4,245,021. Typical fluorene charge transport molecules include 9-(4xe2x80x2-dimethylaminobenzylidene)fluorene, 9-(4xe2x80x2-methoxybenzylidene)fluorene, 9-(2,4xe2x80x2-dimethoxybenzylidene)fluorene, 2-nitro-9-benzylidene-fluorene, 2-nitro-9-(4xe2x80x2-diethylaminobenzylidene)fluorene and the like.
4. Oxadiazole transport molecules such as 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, imidazole, triazole, and others as described in German Patents Nos. 1,058,836, 1,060,260 and 1,120,875 and U.S. Pat. No. 3,895,944.
5. Hydrazone transport molecules including p-diethylaminobenzaldehyde-(diphenylhydrazone), p-diphenylaminobenzaldehyde-(diphenylhydrazone), o-ethoxy-p-diethylaminobenzaldehyde-(diphenylhydrazone), o-methyl-p-diethylaminobenzaldehyde-(diphenylhydrazone), o-methyl-p-dimethylaminobenzaldehyde(diphenylhydrazone), p-dipropylaminobenzaldehyde-(diphenylhydrazone), p-diethylaminobenzaldehyde-(benzylphenylhydrazone), p-dibutylaminobenzaldehyde-(diphenylhydrazone), p-dimethylaminobenzaldehyde-(diphenylhydrazone) and the like described, for example, in U.S. Pat. No. 4,150,987. Other hydrazone transport molecules include compounds such as 1-naphthalenecarbaldehyde 1-methyl-1-phenylhydrazone, 1-naphthalenecarbaldehyde 1,1-phenylhydrazone, 4-methoxynaphthlene-1-carbaldehyde 1-methyl-1-phenylhydrazone and other hydrazone transport molecules described, for example, in U.S. Pat. Nos. 4,385,106, 4,338,388, 4,387,147, 4,399,208 and 4,399,207. Yet other hydrazone charge transport molecules include carbazole phenyl hydrazones such a 9-methylcarbazole-3-carbaldehyde-1,1-diphenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1-methyl-1-phenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1-ethyl-1-phenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1-ethyl-1-benzyl-1-phenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1,1-diphenylhydrazone, and other suitable carbazole phenyl hydrazone transport molecules described, for example, in U.S. Pat. No. 4,256,821. Similar hydrazone transport molecules are described, for example, in U.S. Pat. No. 4,297,426. Preferred hydrazone transport molecules include derivatives of aminobenzaldehydes, cinnamic esters or hydroxylated benzaldehydes. Exemplary amino benzaldehyde-derived hydrazones include those set forth in the Anderson et al U.S. Pat. Nos. 4,150,987 and 4,362,798, while exemplary cinnamic ester-derived hydrazones and hydroxylated benzaldehyde-derived hydrazones are set forth in the copending Levin et al U.S. applications Ser. Nos. 08/988,600, now abandonded, and 08/988,791, now U.S. Pat. No. 5,925, 486, respectively, all of which patents and applications are incorporated herein by reference.
The charge transport layer typically comprises the charge transport compound in an amount of from about 5 to about 60 weight percent, based on the weight of the charge transport layer, and more preferably in an amount of from about 15 to about 40 weight percent, based on the weight of the charge transport layer, with the remainder of the charge transport layer comprising the binder, and any conventional additives.
The charge generation layer typically comprises binder and a charge generation compound. The polymeric binder of the charge generation layer may be any polymeric binder known in the art for use in charge generation layers, including any of the binders noted above for use in the charge transport layer. Various charge generation compounds which are known in the art are suitable for use in the charge generation layer of the photoconductors according to the present invention. Organic charge generation compounds are suitable for use in the present photoconductors, examples of which include, but are not limited to, disazo compounds, for example as disclosed in the Ishikawa et al U.S. Pat. No. 4,413,045, tris-azo and tetrakis-azo compounds as known in the art, phthalocyanine dyes, including both metal-free forms such as X-form metal-free phthalocyanines and the metal-containing phthalocyanines such as titanium-containing phthalocyanines as disclosed in U.S. Pat. Nos. 4,664,997, 4,725,519 and 4,777,251, polymorphs and derivatives thereof, and squaric acid-derived dyes, for example hydroxy-squaraine charge generation compounds. Both metal-free forms and metal-containing forms of the phthalocyanines are preferred. A preferred charge generation compound for use in the charge generation layer according to the present invention comprises metal-containing phthalocyanines, and more particularly metal-containing phthalocyanines wherein the metal is a transition metal or a group IIIA metal. Of these metal-containing phthalocyanine charge generation compounds, those containing a transition metal such as copper, titanium or manganese or containing aluminum as a group IIIA metal are preferred. The metal-containing phthalocyanine charge generation compound optionally may be oxy, thiol or dihalo substituted. Oxo-titanyl phthalocyanines are especially preferred, including various polymorphs thereof, for example type IV polymorphs, and derivatives thereof, for example halogen-substituted derivatives such as chlorotitanyl phthalocyanines.
The charge generation compounds are employed in the charge generation layer in conventional amounts suitable for providing the charge generation effects. Suitably, the charge generation layer comprises at least about 5 weight percent, based on the weight of the charge generation layer, of the charge generation compound, and preferably at least about 10 weight percent, based on the weight of the charge generation layer. In further preferred embodiments, the charge generation layer comprises at least about 15 weight percent of the charge generation compound and preferably from about 15 to about 50 weight percent of the charge generation compound, based on the weight of the charge generation layer, with the balance comprising binder and optionally, conventional additives.
The charge generation and charge transport layers may be formed on the photoconductor substrate according to the present invention in accordance with customary techniques in the art. Typically, the respective charge generation layer and charge transport layer are formed by dispersing and/or dissolving the respective charge generation compound and charge transport compound in a polymeric binder and solvent, coating the dispersion and/or solution on the respective underlying layer, and drying the coating. The charge generation layer will typically have a thickness of about 0.05 to about 5.0 microns while the charge transport layer will typically have a thickness of from about 10 to about 40 microns.